There are numerous difficulties in setting up optical connections in a multi-domain Wavelength Division Multiplex (WDM) optical network. Multi-Domain means that there are interconnected optical network domains. Each optical network domain is operated by a different administrative entity; typically each administrative entity is a telecommunications carrier. A multi-domain optical network requires setting up point-to-point optical connections that have their end-points in different domains. One important goal is to be able to set up end-end multi-domain connections very quickly (e.g., ranging from 100 ms to a few seconds). Another important goal is that within each domain it is desired to have the connectivity be as much all-optical as possible. However, the optical channel interconnection between domains must go through Optical-Electrical-Optical (OEO) processing on each end of a link connecting two domains. This is because the OEO functionality optically isolates the two domains from one another. Within a domain, OEO is used to do wavelength conversion or regeneration as described below. The OEO functionality at one end of a link is provided by two back-to-back transponders. The optical connection between domains is called an External Network Node Interface (E-NNI) and has been standardized by the Optical Internetworking Forum (OIF).
Within a domain, the optical network consists of optical switches, fiber connecting the optical switches, and WDM technology used to carry multiple wavelengths (optical channels) in a fiber. The optical switches are either Reconfigurable Optical Add-Drop Multiplexers (ROADMs) or Optical Cross Connects (OXCs). ROADMs can be viewed as small OXCs (i.e., they connect to a small number of fibers). ROADMs and OXCs have add/drop ports that connect to client ports, and optical connections between client add/drop ports are set up through the ROADM and OXC optical switching fabrics. An optical connection is set up through multiple fibers. A basic connection within a domain consists of a single wavelength channel, and the frequency of the wavelength channel is the same frequency in each fiber the connection goes through. The ROADMs and OXCs cross-connect the wavelength used by the connection from one fiber to the other. In order for a single wavelength to be used from domain border node to domain border node for a connection, there must be a fiber path between the domain border nodes that has that wavelength available on each fiber in the path (i.e., it is not being used for another connection on any of the fibers along the fiber path). This is known as the “Wavelength Continuity Constraint” (WCC).
If within a domain a single wavelength is not available in each fiber along a fiber path, the connection can be established using wavelength conversion (OEO) within the ROADMs or OXCs connecting two fibers that require different wavelengths. However, not every node will support dynamic provisioning of transponders, so wavelength conversion can only be done at nodes equipped with transponder pools. It is desirable to minimize the amount of wavelength conversion required, since the transponders used to do the wavelength conversion are expensive. A wavelength conversion requires two back-to-back transponders. Thus, an important part of setting up optical connections within a domain is having information available to be able to determine what wavelengths are available in the different fibers and what OXCs/ROADMs have available transponders to do the OEO wavelength conversion. This information is needed to efficiently set up optical connections within a domain.
Another aspect of setting up optical connections is that some services provide restoration or 1+1 protection after a failure (e.g., fiber cut) causes the working channel to fail. One type of restoration is known as end-to-end “Shared Mesh Restoration.” In this method of restoration an end-to-end restoration path that is diverse from the working path is determined as part of the connection provisioning process. The restoration paths are only set up after a failure occurs, so if two working connections do not share any failure modes, they can both “share” the same restoration resources. Thus, for provisioning connections using shared mesh restoration, it is important to be able to identify what wavelengths on different fibers can be shared for restoration.
In 1+1 protection a working path and a diverse, dedicated protection path are set up. When a failure occurs on the working path, both ends of the connection switch to the diverse protection path. Thus, when a 1+1 connection is set up, both a working path and a diverse protection path must be determined and set up.
Another important aspect of the problem of setting up optical connections is that routing must be done to determine what route a connection will take (if a shared mesh restoration path or 1+1 protection path is to be provided, then that diverse restoration/protection path must also be determined). It must be determined what domains will be used to provide the connection (and restoration/protection) path, what border nodes will be used to go in and out of each domain, and what route will be used in each domain to connect the chosen border nodes (or a connection end node and a border node). An important constraint is that network operators want to keep their network topology and capacity usage information private, so detailed routing information (network topology and network state) cannot be shared across domains, and detailed routing information within a domain must be generated and kept within that domain.
There have been previous approaches to routing and setting up connections in a multi-domain optical network. There are two basic approaches that have been used to determine the routing of the working and restoration/protection paths. One is called the Per-Domain Approach and the other is called the Backward Recursive Path Computation (BRPC) approach. In either approach the methods assume that for a particular source-destination node-pair there is a pre-determined sequence of domains (Domain 1, 2, . . . N) and candidate domain border nodes that will be used for determining the working paths.
Paths are computed using link cost metrics, and minimum cost paths are computed. For working paths the metric is typically based on the latest recorded spare capacity on the link and link weighting factors the carrier assigns to the link. The link routing information is sometimes distributed by a domain routing protocol such as OSPF-TE (IETF RFC 4203). Route computation can either be done by the domain border nodes or by a Path Computation Element (PCE). In multi-domain networks a PCE is usually used since it can more easily communicate with other domain PCEs. For the previous methods described here, all routing is done by PCEs.
In the per-domain approach the path computation is done one domain at a time, starting at the source node. First a minimum cost working path from the source node to each of the candidate domain border nodes is determined, and the border node having the least cost path is chosen. This determines the egress border node from the source domain and the candidate ingress border nodes in the next domain that will be used. In the next domain, an optimal path from each candidate ingress border node to each candidate egress border node is determined. The minimum cost ingress node to egress node path is chosen for traversing that domain. This process is continued from domain to domain until the destination domain is reached. At the destination domain, a minimum cost path is determined from the ingress border node to the destination node. Note that this methodology does not necessarily find the minimum cost end-to-end path.
In the BRPC approach, the process begins at the destination node in Domain N. Minimum cost paths are computed from each potential ingress border node to destination D. This gives a Virtual Shortest Path Tree (VSPT) between the border nodes of Domain N and Destination D. This VSPT is attached to the candidate egress border nodes of the previous domain, Domain (N−1). Minimum cost paths from each ingress border node of Domain (N−1) to destination D are then computed. This gives a VSPT from Domain (N−1) ingress nodes to destination D. This is done recursively until a minimum cost path from the source to destination is determined.
RSVP-TE signaling, extended for GMPLS (RFC 3473), is used to establish the connections, which includes determining what wavelengths will be used and where wavelength conversion needs to be done. This is done after the routing has been done, so the routing decisions described above do not consider wavelength conversion requirements.
The prior solutions, however, has failed to completely solve the problem of setting up optical connections in a multi-domain network. In previous solutions, for each connection request a multi-domain routing function must be performed to determine the working path and, when restoration or 1+1 protection is provided, a restoration/protection path. After that routing work is done, signaling must be done to set up the working path. In the previous methods, this routing function takes significant time (e.g., hundreds of ms) and would not be able to meet setup times on the order of 100 ms.
Most of the previous solutions do not include providing shared restoration or 1+1 protection capability. One prior approach attempts to enhance the BRPC approach to include computation of a 1+1 protection path. Also, the 1+1 method used in previous work requires the protection path to go through the same domains as the working path.
Another prior approach uses solutions that provide shared restoration within the domains, but this methodology does not protect against failure of border nodes or failure of links connecting border nodes (i.e., links connecting two domains). Thus, additional capabilities are needed to provide full protection of working connections.
None of the previous solutions address the optimization of the use of wavelength conversion within a domain when the Wavelength Continuity Constraint (WCC) cannot be satisfied end-to-end within a domain. For example, one prior solution blocks connection requests when the WCC cannot be met. It is widely recognized that wavelength conversion capability is essential in optical networks in order to get efficient use of the optical resources. However, wavelength conversion requires expensive transponders, and so it is highly desirable to have provisioning methods that can minimize the amount of wavelength conversion required.
A multi-domain optical network provisioning methodology that keeps resource information private in each domain, but optimizes connection setup equivalent to full information sharing across domains is disclosed.